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SCIENCE PROGRESS 



(2) A soda series, with a predominant soda-felspar (anortho- 

 clase, soda-orthoclase, albite) ; or with a soda-lime felspar in 

 combination with nepheline, and one or more soda-pyroxenes 

 and soda-amphiboles. 



(3) A felspathoid series ', in which nepheline, sodalite, or allied 

 felspathoid is largely developed, and felspar subordinate. 



Although in nature there is a perfect gradation in each of 

 these series from a most acid to a most basic type, yet for 

 classificatory purposes it is convenient to break them up into 

 three divisions corresponding to the Acid, Intermediate, and 

 Basic groups as above defined. By doing this the following 

 scheme of classification is obtained : 



Scheme of Classification of the Plutonic Rocks 



In the above scheme there are six main divisions or 

 families, namely : 



1 The nepheline in these families may, of course, be partially or wholly 

 represented by leucite, sodalite, or other felspathoid mineral. 



